It has been standard practice for many years to use a slip agent to reduce the coefficient of friction (COF) of the surface of polypropylene films. By this means, blocking of adjacent layers of film on mill rolls is prevented and the film can readily be unwound for use or for further processing. In the absence of the slip agent, the layer-to-layer COF is so great that smooth unwinding of the film is virtually impossible. Typical slip or antiblock agents are the higher molecular weight fatty acid amides such as stearamide or oleamide. It has become standard practice in the art to incorporate about 0.1 to 1%, preferably 0.2 to 0.5% by weight of such compounds into polyolefin films as slip agents.
In another aspect of the polyolefin film business, there is a rapidly growing market for films carrying a metal coating on one or both surfaces, either for functional or aesthetic reasons.
The films employed for this purpose are usually composites consisting of a polypropylene core layer having, on at least one of its surfaces, a relatively thin layer of a propylene--ethylene copolymer. This copolymer layer is subjected to a corona discharge treatment and the coating metal is then deposited by such methods as electroplating, sputtering or vacuum metallizing. Here, however, the problem is encountered that the presence of conventional concentrations of slip agent in the film destroys the adhesion of the metal coating to the film. In consideration of this problem, it has been proposed to prepare metallizable films by removing the slip agent from the film completely. Such films have, however, not been particularly popular on a commercial scale due to the high COF of the film surfaces and resultant blocking of the film upon unwinding due to the lack of slip agent.
It is the purpose of this invention to provide a metallizable polypropylene film which is not subject to the problems set forth hereinabove.